Sound attenuating laminate for jet aircraft engines

ABSTRACT

The present invention relates to a novel sound attenuating laminate. More particularly, the invention relates to a simpler, lighter, and more effective noise attenuating laminate which is made up of seven layers of material. The laminate includes a duct liner, a moisture barrier, a first protecting layer, a screen, acoustic attenuating material, a second protecting layer, and a solid backing sheet. The laminate is readily incorporated into various sections of a jet engine compartment in order to attenuate the sound produced by the jet engine. Hollow rivets are used to conduct acoustical energy to the intermediary layers of the noise attenuating laminate.

This application is a continuation-in-part of U.S. Ser. No. 106,618,filed October 6, 1987, U.S. Pat. No. 4,848,514 and entitled SoundAttenuation System for Jet Aircraft Engines.

The present invention relates to the field of sound attenuation, andmore particularly to a sound attenuating laminate which can be used forthe suppression of sound produced by jet aircraft engines.

BACKGROUND OF THE INVENTION

The method presently in use for attenuating noise generated by thecommercial jet aircraft engines was initially developed in the late1960's and early 1970's by the manufacturers of such aircraft. Thistechnology consists of an acoustic lining system for the engine, havinga sandwich-type construction consisting of sintered metal mesh facingthe engine air flow paths in the inlet and fan exit ducts. This mesh isbonded to the duct liner skin with a precisely sized and spaced holepattern tuned to the primary noise frequency generated in that portionof the engine at the critical operating mode being silenced. The metalmesh and perforated duct liner skin are bonded to a honeycomb structurebacked with a solid skin.

At the time this system was developed, the industry objective was tomeet the requirements of Federal Air Regulation Part 36, Stage 2. Thissystem remains the primary industry development for attenuating thenoise generated by narrow-bodied jets, as it has been perceived as theonly system adequate for service in its unique operating environment inthe jet engine. For example, JT3D commercial jet engines installed onBoeing 707 and DC8 aircraft have used and continue to use this system.

Sound attenuation with this system is accomplished by the HelmholzResonator effect whereby cavities in the honeycomb dissipate acousticalenergy after its admittance through the metal mesh and perforated skinwhich has been placed between the honeycomb and the sound generatingelements of the engine. The solid skin backing in the honeycomb isimpervious to acoustical energy radiation and prevents acousticaltransmission. Some structure-borne sound transmission is transmitted bythe sandwich construction, but this is of a secondary nature. Loss inengine performance, however, has been associated with air leakagethrough the honeycomb lining.

The noise generated by the fan section of jet engines occurs at discreteprimary frequencies which vary depending on engine model, fan speed, andlocation along the duct. Attenuation of such noise using the abovementioned method has the potential to achieve the initial goal of andcompliance With Stage 2 of the Federal Air Regulations, Part 36.Therefore, industry research has concentrated on the precise tuning ofthe lining design to the engine noise source characteristics, withemphasis on acoustic parameters of different dimensional and materialproperties of the porous metal facing sheet, honeycomb core, and thesolid backing sheet. Results have been barely adequate with differingdegrees of economic and operational penalties.

The system described above is estimated to be capable of producing anattenuation of 6 to 11 DB and requires the meticulous fine tuning ofcritical parameters such as skin hole size, metal mesh grid andthickness, honeycomb material makeup, and cavity dimension andthickness. This system is designed to attenuate only one primaryfrequency for a given combination of critical parameters. The result hasbeen marginal compliance with the Stage 2 requirements of Federal AirRegulations, Part 36. Moreover, Stage 3 requirements have not yet beenreached with the present system without resort to measures which resultin a high cost and high risk solution to the noise problem and asignificant engine performance penalty, exposure to catastrophic enginefailure, and continuing maintenance problems. A further problem withsuch measures would involve obtaining airframe and engine manufacturerapproval for the attendant inlet airflow changes.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide astraightforward, well-constructed noise attenuating laminate for variousjet aircraft engines which allows for more efficient attenuation ofsOund energy.

It is a further object of this invention to provide a noise attenuatinglaminate for jet aircraft engines which would comply with Stage 3 of theFederal Air Regulations, Part 36 noise attenuation requirements withminimal economic and operational penalties.

It is yet a further object of this invention to provide a soundattenuating laminate which would achieve nonlinear acoustic attenuationover a broad range of noise frequencies without the need for any finetuning requirements.

It is another object of this invention to provide for a soundattenuating laminate which would prevent loss of engine performanceassociated with air leakage.

The objects of this invention are accomplished by providing a soundattenuating laminate which consists of a number of layers. The firstlayer is a fan inlet duct and center body liner, fan exhaust duct innerand outer liner, and engine access door liner. These liners areperforated to allow for the entry of acoustic energy and can consist ofaluminum or stainless steel. The second layer is a moisture barrierwhich is located next to the perforated liner and can be made up offiberglass fabric or fire resistant ceramic fabric. This layer issaturated with pressure sensitive silicone adhesive and thus bonded tothe liners above. The third layer consists of fiberglass coated witheither silicone rubber, viton, fluourosilicone, or teflon. This layerprotects the fourth and fifth layers from moisture, fluids, and othercontaminants and serves as a membrane for the passage of acoustic energyto the acoustic attenuating material. The fourth layer consists of ascreen of plastic, such as polypropylene, or stainless steel. Thisscreen serves as a spacer to separate the third layer from the acousticattenuating material of the fifth layer. This screen also provides afill air space to allow for the passage of acoustic energy to theacoustic attenuating material. The fifth layer consists of acousticattenuating material such as fiberglass or ceramic fiber blanketing. Thesixth layer can be made up of any of the same materials as the thirdlayer. The seventh and last layer is a solid backing sheet which may bethe outer skin of the nose cowl. This layer reflects any acoustic energywhich has passed through the preceding layers back towards the acousticattenuating material.

In installing the laminate, hollow rivets are utilized to replacecertain solid structural rivets at certain locations of the jet engine,most particularly the inlet duct and the nose cone. These hollow rivetspenetrate anti-icing airflow channels to achieve an improved anti-icingsystem and allow for the passage of acoustic energy radiation from itsorigin, through the duct liner or nose cone outer skin, and to theacoustic attenuating material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the seven layers making up anembodiment of the sound attenuating laminate described in the presentinvention;

FIG. 2 is a diagram showing a jet engine nacelle indicating where thesound attenuating laminate described in this invention would be placedwithin such a jet engine nacelle;

FIG. 3 is a diagram showing the installation of the sound attenuatinglaminate in the air inlet duct area of a jet engine nacelle;

FIG. 4 is a diagram showing the installation of the sound attenuatinglaminate in the nose dome area of a jet engine;

FIG. 5 is a diagram showing the use of hollow rivets to allow for thepassage and attenuation of acoustical energy and to improve theanti-icing system.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring specifically to the drawings, FIG. 1 illustrates oneembodiment 10 of the present invention. According to this invention, thesound attenuating laminate has seven layers as can be seen in FIG. 1.The first layer 12 is a liner which may be a fan inlet duct liner, acenter body liner, a fan exhaust duct liner, or an access door liner.This liner 12 can consist of aluminum or stainless steel and containsperforations 13 that allow for the entry of acoustic energy. Theperforations 13 are preferably about one-eighth of an inch in diameterand spaced at approximately five-sixteenth of an inch intervals alongthe liner.

The second layer 14 of the sound attenuating laminate is composed of amoisture barrier. This moisture barrier 14 is adjacent to the liner 12.A suitable moisture barrier material was found to be fiberglass fabricor fire resistant ceramic fabric. These fabrics are saturated withpressure sensitive silicone adhesive such as PSA 52 with SRC 18catalyst, manufactured by the General Electric Company, SiliconeProducts Division, Waterford, N.Y., and are thereby made to bond to theliner 12.

The third layer 16 is a protecting layer and can be composed offiberglass fabric coated with silicone rubber, viton, fluorosilicone, orteflon. This layer protects the fourth and fifth layers from vapor,moisture, fuel, hydraulic fluid, engine oil, and other contaminateswhich would be hazardous if allowed to collect and which would alsodetract from the performance of the acoustic attenuating material. Thethird layer 16 also serves as a gas permeable membrane allowing for thepassage of acoustic energy to acoustic attenuating material in the fifthlayer 20. Acoustical energy is carried by air molecules which impinge onthe gas permeable membrane 16 causing it to vibrate at the frequency ofthe acoustical energy being carried. As the gas permeable membrane 16vibrates, it in turn energizes air molecules on its other side,directing them to the acoustic attenuating material.

The fourth layer 18 consists of a screen of stainless steel or plastic,such as polypropylene, with openings of approximately one eighth of aninch which serves as a spacer to separate the third layer 16 from thefifth layer 20 while maintaining an air space between the third andfifth layers so as to allow the fiberglass coated fabric of the thirdlayer 16 to perform as a membrane in allowing acoustic energy to passthrough to the acoustic attenuating material of the fifth layer 20.

The acoustic attenuating material in the fifth layer 20 is formed in ablanket type configuration, preferably two inches thick and is adjacentto the screen of the fourth layer 18. One suitable sound attenuatingmaterial was found to be Kaowool ceramic fiber blanketing, manufacturedby the Insulating Products Division of Babcock and Wilcox, a McDermottcompany. Another suitable acoustic attenuating material was found to be1000 Series Spin-Glas Fiber Glass Insulation, manufactured by theManville Building Materials Corporation, a subsidiary of the ManvilleCorporation, Ken-Caryl Ranch, P.O. Box 5108, Denver, Colo. 80217. Asdiscussed below, such materials are appropriate for use in differentportions of the engine.

The sixth layer 22 is also a protecting layer and can be made up of thesame materials as the third layer 16. The surface of the sixth layer 22facing away from the acoustic attenuating material of the fifth layer 20may be additionally coated with aluminum foil to serve as a radiationheat shield where elevated temperatures make this necessary.

The seventh layer 24 is a solid backing sheet which can be composed ofaluminum or titanium and which may actually be the outer skin of thenose cowl, fan ducts, or cowl doors, or the inner surface of the nosecowl. It serves to reflect any acoustic energy which has passed throughthe preceding layers back towards the acoustic attenuating material inthe fifth layer 20.

As one can see from the previous discussion, a number of variations arepermitted in the construction of the novel laminate. For example, a fireresistent layer could be added to the sound attenuating laminate betweenthe second layer 16 and the third layer 18 in order to protect theacoustic attenuating material and the external aircraft structure fromfire hazards. A suitable fire barrier was found to be Nextel 312 WovenFabric, manufactured by the Ceramic Materials Department of the 3MCorporation, Building 225-4N-07, 3M Center, St. Paul, Minn. 55144-1000.Furthermore, the third layer 16 and sixth layer 22 of the soundattenuating laminate could be fused together at their edges by heat(vulcanized), or by bonding with an adhesive such as PSA52 with SRC 18catalyst manufactured by the General Electric Company, Silicone ProductsDivision, Waterford, N.Y., thereby encapsulating the fourth 18 and fifthlayer 20 further protecting them from vapor, moisture, fuel, hydraulicfluid, engine oil, and other contaminants.

The resulting laminate is incorporated into various portions of the jetengine compartment. FIG. 2 shows a typical jet engine nacelle 30, suchas that used to house the commercial model JT3D jet engine. Shaded areas31-36 show the application of the laminate of the present invention.Primary areas for application include the fan inlet duct 44, the nosedome 42, the fan exhaust ducts 46 and access doors to the nacelle whichenvelop the engine around its circumference from the nose cowl aft tothe exhause duct.

In the more forward areas of the engine, it is preferred to employ afiberglass material as the acoustic attenuating material of the fifthlayer 20, (e.g., areas 31, 32), while in more rear-ward areas, such asareas 34, 35 and 36, it is preferred to use the ceramic fiber material.The fiberglass material, however, is a superior sound attenuatingmaterial which is substantially lighter in weight. The ceramic fibermaterial is used adjacent to hot areas of the engine as it willwithstand temperatures in excess of 2000° F.

The sound attenuating laminate of the invention incorporates theexisting air flow liner in all areas except the access doors. The solidbacking sheet can be the outer skin of the nacelle. For example, theinner surface of the access door outer skin serves as the backing sheetin that location. A perforated sheet of aluminum or stainless steel willface the engine, with the other layers of the sound attenuating laminatebeing placed between the perforated sheet and the inner surface of theouter skin. The perforated sheet will be mechanically fastened to theadjacent structure of the door.

For example, FIG. 3 shows the installation of the sound attenuatinglaminate 10 in the air inlet duct area of a jet engine nacelle. Theintermediary layers of laminate 10 are installed around the air inletduct 44, between its structural ribs 60. Once wound, the intermediaryfive layers 14-22 of the laminate are secured with velcro tapes 62. Theperforated layer 12 is installed inside the inlet duct 44 and the outerskin 64 of the inlet duct 44 acts as the solid backing layer 24.

FIG. 4 shows the installation of the sound attenuating laminate in thenose dome area of a jet engine. The laminate is installed within thenose bullet aft section 70, with hollow rivets 72 allowing the acousticenergy impinging on the nose cone to enter the intermediary layers 14-22of the sound attenuating laminate.

The fan inlet duct areas are equipped with an anti-icing system,consisting of channels in the underside of each such structure throughwhich hot air flows during anti-icing system operation. Such channelsare composed of corrugations formed by the riveting of an inner liner tothe inner wall of the fan duct inlet. This inner liner has been upset toform corrugated channels on the back surface of the inlet liner. Wheninstalling the laminate of the present invention, the existing solidrivets will be replaced with hollow rivets, and additional hollow riVetsadded as shown in FIG. 5. As seen there, the anti-icing channels 56 areformed by corrugated sheet 52 abutting a wall 54, which could be thewall of the fan inlet duct.

Conventionally, rivets have been placed at positions such as 58, wherethe corrugated sheet makes contact with the wall. In the presentinvention, however, not only are the rivets 58 replaced by hollowrivets, but additional hollow rivets 59 are inserted through channels56. These rivets act as a novel means for providing conduits in the ductliner for the passage of acoustic energy generated by the jet enginethrough the wall of the fan duct or nose dome to the sound attenuatinglaminate. Furthermore, these hollow rivets will serve as acoustic hornswhich will not only strengthen the anti-icing duct nacelle throughstructural support, but will also act as efficient and easily installedconduits for conducting acoustic energy.

The duct liner and an inner liner will be riveted together to form astructurally stiff corrugated structure. This structure will also formthe air flow passages for the flow of hot anti-icing air from thecowling inlet lip along the length of the inlet duct. The hollow rivetswill function primarily for the passage of acoustic energy radiation.The rivets will also intersect the air flow passage at a right angle tothe flow, and thus will also function as heat exchange baffles toimprove the efficiency of the anti-icing systems of the inlet duct.

For example, the nose dome 42 of the engine normally has insertsattached to the dome's inner surface forming anti-icing air flowchannels. These channels will be replaced in the present design. Asshown in FIG. 4, the aft section 70 of the nose dome Will be perforatedto allow passage of acoustic energy to the sound attenuating laminateinstalled in the center of the dome's aft section 70. Hollow rivets 72will be installed through the anti-icing channels 74 of the nose dome.

Efficiency of the jet engine anti-icing system will be improved in twoways. First, the hot air impinging on the outer surface of the hollowrivet will readily transmit heat to the duct liner outer skin. Second,the hollow rivets penetrating the anti-icing air passages will slow airflow through the passages, thus allowing for the transfer of more of theheat content of the hot air to the metal surfaces. The improvement inthe anti-icing system efficiency may allow the jet engine anti-icingsystem to operate at reduced amounts of hot anti-icing bleed air, thusreducing the amount of engine air bled off for this purpose. Enginethrust may therefore be increased and fuel efficiency improved duringanti-icing system operation.

The sound attenuating system of this design has been demonstrated toattenuate a broad range of noise frequencies without special finetuning. It is estimated that attenuations of up to 23 DB may be possiblebased on previous experimental work. Furthermore, this system meets theprimary requirements for fire and heat resistanCe imposed by the FederalAviation Administration. This laminate system will be sealed from theengine air flow using the moisture barrier lamination to prevent the 2%loss in engine performance associated with the older sandwich typeconstruction. The system will also be sealed from moisture and liquids,another significant problem inherent with the older sandwich-typeconstruction.

In the foregoing specification, the invention has been described withreference to a specific exemplary embodiment thereof. It will, however,be evident that various modifications and changes may be made thereuntowithout departing from the broad spirit and scope of the invention asset forth in the appended claims. The specification and drawings areaccordingly to be regarded in an illustrative rather than in arestrictive sense.

What is claimed is:
 1. A multilayered laminate having improved soundenergy attenuating properties comprising:a perforated facing layer; amoisture barrier layer adjacent to said facing layer; a first protectinglayer adjacent to said moisture barrier layer; a screen layer adjacentto said first protecting layer; an acoustic attenuating material layeradjacent to said screen layer; a second protecting layer adjacent tosaid acoustic attenuating material layer; and a solid backing sheetlayer adjacent to said second protecting layer.
 2. The article of claim1, wherein the perforations in said facing layer are comprised of aplurality of hollow rivets disposed throughout said facing layer.
 3. Thearticle of claim 1, wherein said perforated facing layer is composed ofaluminum.
 4. The article of claim 1, wherein said perforated facinglayer is composed of stainless steel.
 5. The article of claim 1, whereinsaid moisture barrier layer is composed of fiberglass fabric saturatedwith pressure sensitive silicone adhesive.
 6. The article of claim 1,wherein said moisture barrier layer is composed of fire resistantceramic fabric saturated with pressure sensitive silicone adhesive. 7.The article of claim 1, wherein said first protecting layer is composedof silicone coated fiberglass fabric.
 8. The article of claim 1, whereinsaid first protecting layer is composed of viton coated fiberglassfabric.
 9. The article of claim 1, wherein said first protecting layeris composed of fluorosilicone coated fiberglass fabric.
 10. The articleof claim 1, wherein said first protecting layer is composed of a tefloncoated fiberglass fabric.
 11. The article of claim 1, wherein saidscreen layer is composed of stainless steel.
 12. The article of claim 1,wherein said screen layer is composed of polypropylene.
 13. The articleof claim 1, wherein said acoustic attenuating material layer is composedof ceramic fiber blanketing.
 14. The article of claim 1, wherein saidacoustic attenuating material layer is composed of fiber glassblanketing.
 15. The article of claim 1, wherein said second protectinglayer is composed of silicone coated fiberglass fabric.
 16. The articleof claim 1, wherein said second protecting layer is composed of vitoncoated fiberglass fabric.
 17. The article of claim 1, wherein saidsecond protecting layer is composed of fluorosilicone coated fiberglassfabric.
 18. The article of claim 1, wherein said second protecting layeris composed of teflon coated fiberglass fabric.
 19. The article of claim1, wherein said second protecting layer is coated with aluminum foil.20. The article of claim 1, wherein said solid backing sheet layer iscomposed of aluminum.
 21. The article of claim 1, wherein said solidbacking sheet layer is composed of titanium.
 22. The article of claim 1,wherein said first protecting layer and second protecting layer eachdefine its corresponding first edge and second edge, said first andsecond protecting layers being fused to each other at said first andsecond edges for encapsulating said screen layer and said acousticattenuating material layer.
 23. A multilayered laminate having improvedsound energy attenuating and flame retardant properties comprising:aperforated facing layer; a moisture barrier layer adjacent to saidfacing layer; a fire resistant layer adjacent to said moisture barrierlayer; a first protecting layer adjacent to said fire resistant layer; ascreen layer adjacent to said first protecting layer; an acousticattenuating material layer adjacent to said screen layer; a secondprotecting layer adjacent to said acoustic attenuating material layer;and a solid backing sheet layer adjacent to said second protectinglayer.
 24. The article of claim 23, wherein said fire resistant layer iscomposed of woven ceramic fabric.
 25. The article of claim 23, whereinsaid first protecting layer and second protecting layer each define itscorresponding first edge and second edge, said first and secondprotecting layers being fused to each other at said first and secondedges for encapsulating said screen layer and said acoustic attenuatingmaterial layer.